feat(wasm): sequential rayon shim for the web build

wasm32 has no threads, so rayon's global pool would panic at runtime.
Add `crate::par::prelude`: native re-exports `rayon::prelude`, wasm
provides sequential `std`-iterator equivalents (par_iter / into_par_iter
/ flat_map_iter over slices, Vec, HashMap, HashSet). All call sites now
`use crate::par::prelude::*;` instead of rayon directly.

- wire_gpu's rayon `reduce` (2-arg, no std equivalent) is cfg-split to
  a sequential flat_map concat on wasm.
- rayon moves to a non-wasm target dependency.

Native still runs in parallel; all 75 tests pass. wasm bin compiles.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/mod.rs

@@ -155,7 +155,7 @@ pub fn build_derived_caches(doc: &CadDocument) -> DerivedCaches {
     // filters that `cache::block_cache::build_defn` already uses for block defns)
     // and prefer the entity-scan when the header center drifts more than
     // 10× its own half-span away from the entity centroid.
-    use rayon::prelude::*;
+    use crate::par::prelude::*;
 
     // Single pass over entities does triple duty: classify cache-kind handle
     // lists (hatch / image / mesh) AND accumulate per-entity centroids for the
@@ -2245,7 +2245,7 @@ impl Scene {
         };
 
         // Tessellate in parallel across all available CPU cores.
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         let doc = &self.document;
         // Selection / hover highlight is NOT baked into tessellation. It is
         // applied per frame in the GPU xray overlay pass from the live
@@ -4297,7 +4297,7 @@ impl Scene {
             })
             .collect();
 
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         self.hatches = entries
             .into_par_iter()
             .filter_map(|(handle, kind)| {
@@ -4350,7 +4350,7 @@ impl Scene {
             })
             .collect();
 
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         let facet_res = self.document.header.facet_resolution;
         let woff = self.world_offset;
         self.meshes = entries